Potential of hazardous waste encapsulation in concrete with coal fly ash and bivalve shells

Coal fly ash (FA) is abundant in countries that use steam coal for power production. The resultant FA is Class F FA (<20% CaO, Al₂O₃ + SiO₂ > 70%, <6% Loss on Ignition) and can be used as an excellent scrubber and fixation reagent for acidic wastes containing toxic trace elements. In Spain, less than 50% of the abundant FA is currently being used. This paper reports on the potential use of Class F FA as a neutralization and fixation reagent for the hazardous sludge waste of the phosphate industry. Several analytical techniques (ICP-MS, ICP-AES, SEM-EDX, and XRD), and leaching experiments (EN12457-2) were used for this study. The results identify the best FA/waste ratios for full neutralization and fixation. Furthermore, adding a high calcium carbonate material prevents the excessive leaching of As, Mo, and Se, and facilitates full neutralization and fixation. Furthermore, the use of a safe aggregate (produced by mixing coal phosphate waste with fly ash and/or bivalve shells) as a partial substitute for sand improves the quality of concrete for civil engineering projects and is environmentally safe.